Process control systems often employ pneumatic actuators to operate fluid valves as well as other process control devices. Pneumatic actuators typically include a spring casing having upper and lower casing halves between which a diaphragm is captured. An actuator shaft or rod is typically coupled to the diaphragm so that movements of the diaphragm cause corresponding movements of the actuator rod. In turn, if the actuator rod is coupled to, for example, a fluid valve, the movements of the actuator rod may be used to control the position of a fluid flow control member (e.g., a plug) within the valve and, thus, the fluid flowing through the valve.
One or more springs within the spring casing bias the diaphragm and the actuator rod toward a known position and pressurized air may be applied to one side of the diaphragm via a port in one of the casing halves to move the diaphragm and actuator rod against the forces applied by the spring(s). Thus, the springs provide a return force to enable bi-directional movement and control (e.g., open/close control, modulating control, etc.) of the diaphragm and actuator rod positions via a single pressure signal to the actuator.
Typically, the spring casing of a pneumatic actuator includes upper and lower casing halves that are made of stamped, forged, or cast metal. Each of the casing halves typically has an internal cavity with a depth or height that enables the assembled casing halves to accommodate the height of the springs in a desired biased (i.e., partially compressed) condition within the assembled spring casing. Thus, as larger, more powerful springs are needed to satisfy certain applications, the length of the springs needed tends to increase, which requires an increased depth or height of the internal cavities within the casing halves. However, simply increasing the depth or height of the internal cavities within the casing halves can be problematic. For example, in the case where a stamping, forging, or casting process is used to fabricate casing halves, the depth of the casing halves cannot be increased without practical limitation. In particular, as the desired depth of a casing increases, the cost of manufacturing the casing may increase and may become cost prohibitive. Alternatively or additionally, beyond a certain casing depth, it may not be possible to use a stamping, forging, or casting process to fabricate the casing halves. For example, beyond a certain depth or height, it may not be possible to easily remove a casing half from the tool used to fabricate the casing half. In other words, the tool may be jammed when the casing half becomes stuck on the tool.